I. Field of the Invention
This invention relates generally to an intravascular deliverable stent for reinforcing a blood vessel, and more particularly to such a stent specifically designed for addressing an abdominal aortic aneurysm (AAA).
II. Discussion of the Prior Art
An aortic aneurysm is a weak area in the aorta, the main blood vessel that carries blood from the heart to the rest of the body. The aorta extends upwards from the heart in the chest and then arches downwards, traveling through the chest (the thoracic aorta) and into the abdomen (the abdominal aorta). The normal diameter of the abdominal aorta is about one inch.
As blood flows through the aorta, the weak area bulges like a balloon and can burst if the balloon gets too big.
Most commonly, aortic aneurysms occur in the portion of the vessel below the renal artery origins. The aneurysm may extend into the vessel's supplying the hips and pelvis.
Once an aneurysm reaches 5 cm in diameter, it is usually considered necessary to treat to prevent rupture. Below 5 cm, the risk of the aneurysm rupturing is lower than the risk of conventional surgery in patients with normal surgical risks. The goal of therapy for aneurysms is to prevent them from rupturing. Once an AAA has ruptured, the chances of survival are low, with 80-90 percent of all ruptured aneurysms resulting in death. These deaths can be avoided if the aneurysm is detected and treated before it ruptures.
Most aortic aneurysms occur in the abdominal aorta, the main cause being arteriosclerosis. This is a condition in which fatty deposits are laid down in the walls of the arteries, which are less elastic and weaker as a result. Major risk factors for arteriosclerosis are smoking and high blood pressure as well as genetic factors.
AAA can be diagnosed from their symptoms when they occur, but this is often too late. They are usually found on routine physical examination and chest and abdominal X-rays. On examination, a doctor may feel a pulsating mass in the abdomen. If the doctor suspects an aneurysm, he/she will probably request that an ultrasound scan be carried out. Other scans, such as computerized tomography (CT) and magnetic resonance imaging (MRI) may also be performed. These scanning techniques are very useful for determining the exact position of the aneurysm.
The surgical procedure for treating AAA involves replacing the affected portion of the aorta with a synthetic graft, usually comprising a tube made out of an elastic material with properties very similar to that of a normal, healthy aorta. This major operation is usually quite successful with a mortality of between 2 and 5 percent. The risk of death from a ruptured AAA is about 50%, even during surgery.
More recently, instead of performing open surgery in undertaking aneurysm repair, vascular surgeons have installed an endovascular stent/graft delivered to the site of the aneurysm using elongated catheters that are threaded through the patient's blood vessels. Typically, the surgeon will make a small incision in the patient groin area and then insert a delivery catheter containing a collapsed, self-expanding or balloon-expandable stent/graft to a location bridging the aneurysm, at which point the stent/graft is delivered out from the distal end of the delivery catheter and allowed or made to expand to approximately the normal diameter of the aorta at that location. The stent/graft, of course, is a tubular structure allowing blood flow through the lumen thereof and removing pressure from the aneurysm. Over time, the stent/graft becomes endothelialized and the space between the outer wall of the stent and the aneurysm ultimate fills with clotted blood. At this time, the aneurysm is no longer subjected to aortic pressures and thus will not continue to grow.
In treating AAA, it is important that the stent be accurately placed so as not to occlude blood flow through the renal arteries which branch off from the abdominal aorta.
In the Amplatz et al. U.S. Pat. No. 6,932,837, there is described a collapsible stent/graft designed for grafting a lumen of a selected blood vessel or other tubular organ. The stent/graft comprises a woven or braided fabric made from a plurality of strands of a shape memory alloy. The fabric is formed as a tube and each end of the device is open to allow fluid flow therethrough. The device can be longitudinally stretched to thereby reduce its diameter, allowing it to be inserted within the lumen of a delivery catheter. When ejected from the distal end of the delivery catheter, the stent/graft will self-expand to a predetermined outer diameter sufficient to engage the wall of the tubular vessel being treated.
While the device in the '837 patent is altogether suitable for use as a coronary stent, it is not well suited for the intravascular treatment of AAA. That device is of a uniform weave, but necessarily is of a wire density that is insufficient to limit the exposure of the aneurysm to aortic blood pressure. Should this stent/graft also encroach upon the ostia of the renal arteries, it will necessarily unduly restrict blood flow to the kidneys.
A need, therefore, exists for a stent/graft that can be placed using an endovascular approach in the treatment of AAA, but that will not unduly occlude blood flow to the kidneys. The present invention provides such a device.